The invention relates to a sheet feeder in which sheets stacked on a hopper are picked to be transported thereinto and the sheets so picked are fed thereinto sheet by sheet by a separator roller and a brake roller which are provided at a separating section, and a jam detecting method therefor.
A related sheet feeder for use with an image reader takes out sheets sheet by sheet by a configuration shown in FIG. 6. In the event that there are a plurality of sheets of document to be read, the plurality of sheets of document are piled up and set in a hopper so that end portions of the sheets so piled up are situated below a pick roller. When the pick roller rotates in such a manner as to pull the sheets so set into the feeder, a sheet existing on the top of the pile of sheets is picked by virtue of a frictional force generated at a portion thereof which comes into contact with the pick roller so as to be fed into a feeder main body. As this occurs, while there occurs a case where not only a single sheet on the top of the pile of sheets of document but also a few sheets from the top of the pile are transported at the same time, the number of sheets to be fed into the feeder is restricted by regulating the thickness of a passable sheet by a feeding gate, and furthermore, only a sheet is designed to be separated from the pile by means of a separator roller and a brake roller so as to be fed into the feeder.
In a sheet feeder like this, in order to detect a jam of a sheet, a sheet detecting sensor is provided in a sheet path existing further ahead of the separator roller, so that a jam is determined on in the event that the passage of the sheet cannot be verified even when a transport means (a roller or a belt, or a vacuum-powered transport device) is operated to produce a distance which allows the sheet to arrive at the sheet detecting sensor based on a comparison between a transport amount of the transport means and a time required for the sheet to arrive at the sheet detecting sensor (refer to JP-A-6-191018).
In addition, there is a method for detecting a jam by detecting the height of a deflected sheet when the sheet continues to be transported although a leading end thereof is stopped due to a jam (refer to U.S. Pat. No. 5,615,876).
A device adopting as a transport means a roller or a belt which make use of a frictional force of rubber or the like is adopted in most sheet feeders. In these sheet feeders, since the separating performance is satisfied by disposing a separating means (a pad and a roller, a belt and the like) in such a manner as to oppositely face a transport means for transporting sheets, a separating resistance is applied to a sheet even when only a single sheet is transported, and, depending on types of sheets and wear state of the roller, and the setting environment of the feeder, there has occurred a difference in displacement is generated due to slippage between the transport means and the sheet.
Consequently, since a delay is generated in sheet detecting time due to the difference in displacement, even when sheets are normally fed without a jam, in the above sheet feeders, a threshold of a drive amount for the transport means, which is based on which a jam is determined on at a jam detecting section has had to be increased to a level several to several tens times an actual transport distance in order not to detect a jam erroneously.
In the event that the above setting is made, however, in case a jam occurs at the sheet feeding section, since the roller is rotated several to several tens times more than the transport distance, there has been caused a problem that when the difference in displacement is small, the sheet is largely deflected and wrinkled, and is, in the worst case, broken.